The SEC said it extended the pilot program through April 21, 2017 to allow more time for evaluation, including how it operated on Aug 24, 2015 when the Dow Jones industrial average slid more than 1,000 points in the first few minutes of trading.

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It said the latest extension also modifies the manner in which some so-called "reference prices" are determined at the start of the day. The SEC said this could help prevent unnecessary trading pauses unrelated to volatility.